Spectrum measurement devices have important application in fields such as optical fiber communication, optical fiber sensing and substance analysis. A volume grating-based spectrum measurement uses a spatial dispersion effect of the volume grating to realize separation of different frequency components of the signal light, thereby completing the measurement of the spectrum of the signal light and the extraction of spectral information. Due to the limitation of factors such as preparation process and device size of the volume grating, the resolution of this type of spectrum measurement device can only reach a magnitude of 1 GHz at present. Since gain bandwidth based on stimulated Brillouin scattering (SBS) effect in an optical fiber is very narrow (at a magnitude of 10 MHz), the SBS effect can equivalently function as a narrowband optical filter which is used for extracting fine spectral components. In combination with a tunable laser, a spectrum measurement device having a resolution at a magnitude of 10 MHz can be formed and has a wide application prospect in the above-mentioned fields.
However, the frequency response of the narrowband optical filter based on the SBS effect changes obviously depending on matching or mismatching of the polarization states of the signal light and the pump light, which results in that the spectrum measurement device based on this effect presents polarization dependence when measuring a signal under test. In other words, with the change of the polarization state of the signal light, the spectrum measured by the device is poor in repeatability and the values of power spectral density corresponding to different frequencies are not stable. As described in the paper (Preussler S, Zadok A, Wiatrek A, et al., Enhancement of spectral resolution and optical rejection ratio of Brillouin optical spectral analysis using polarization pulling.[J]. Optics Express, 2012, 20(13):14734-45), a plurality of polarization controllers are used to realize the matching of the polarization states of the pump light and the signal light, so as to achieve the optimal spectral measurement result. However, when the polarization state of the signal is changed, an accurate spectrum measurement result can only be obtained by adjusting the polarization controllers again. This type of spectral polarization-dependent measurement device based on SBS effect lacks for practical value.